M.W.
I had a brother that would bite so hard his head would shake whild he was biting and could break the skin. My mom tried to punish him but in the end her biting him, hard enough to hurt, did the trick. I don't think he knew that it hurt the other person that bad. I also have girlfriend whose daughter was a terrible bitter, they moved her up to the older kids class in school because they had done everything including biting her back. One day another kid bit her and she finally stopped. Both of them were alot older, so I am not sure a 9 month old would understand that either. Good Luck